P0420 on a 2017 Ford Focus

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2017 Ford Focus

Ranked from 892 NHTSA owner complaints filed for this vehicle.

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1
Catalyst oil contamination from wet-belt/oil-pump failure
High severity
Reported low oil pressure and oil-pump/timing-belt debris events (NHTSA Campaign 23V905000) on this model can push oil into the exhaust stream, coating and poisoning the catalyst washcoat.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Catalyst damage from cracked cylinder head / coolant-intrusion misfires (2.0L EcoBoost)
High severity
Owners report a known 2.0L EcoBoost cylinder head cracking issue that lets coolant into a cylinder and causes misfire; coolant and unburned fuel passing through the exhaust during these events can overheat and damage the catalyst.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Rich/lean fuel trim swings from a failing canister purge valve
Medium severity
Owners report the CPV failing and causing stalling/mixture problems; repeated rich/lean excursions stress and can degrade catalyst efficiency over time.
Reported in 2 NHTSA complaints
4
Aged/worn catalytic converter
Medium severity
Normal high-mileage degradation of the catalyst substrate reduces conversion efficiency below the OBD threshold.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear sensor makes the PCM think the catalyst isn't working even when it is; compare front/rear sensor switching.
Common cause
Parts:oxygen sensor
6
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak between the catalyst and rear sensor introduces outside air, skewing the efficiency calculation.
Common cause

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

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Matching symptoms

Check Engine Light